This invention relates in general to electrical testing apparatus and, in particular, to apparatus for testing personnel grounding systems which are used in the manufacture and handling of static-sensitive electronic devices.
Many advanced electronic devices may be damaged by electrostatic discharge of triboelectric charges on personnel engaged in manufacturing and handling the devices. An electrostatic discharge of 100-150 volts may be sufficient to cause dielectric breakdown in a chargesensitive device such as a complementary metal-oxide-semiconductor (CMOS) integrated circuit. These devices may be exposed to electrostatic charges of several thousand volts which can be generated by the movement of personnel handling the devices.
Consequently, workers involved in the manufacture or assembling of charge-sensitive devices may be required to wear grounding circuits to prevent the buildup of the electrostatic charges on their bodies and clothing. Typically the worker will wear a strap attached to his wrist which will electrically connect his body to a grounding terminal through a large resistance, for example one megohm resistance. This wrist strap discharges the electrostatic buildup before it reaches potentially damaging levels. The resistor is a safety feature isolating the worker from ground in case he comes in contact with high voltage. Shoe/ankle straps are also used to discharge static electric charges through conductive floor plates.
Because the wires of the grounding straps undergo a great amount of movement, they are prone to failure and must be checked periodically to verify that they are operating properly. Typically, they are sent to a testing area where their operation is certified. This, of course, involves a great deal of time and expense. In the past, with a certification period of three months, a failure rate of 10-20 percent has been found in this type of equipment. It is apparent that charge-sensitive devices may be damaged through use of these defective grounding straps and that frequent certification on a weekly or even daily basis is advisable.